1. Field of the Invention
The present invention relates to a hydraulic system for operating components of a machine, and more particularly to such systems for machines, such as a backhoe with a boom assembly for example, in which the hydraulic load varies as a function of the position of the components being operated.
2. Description of the Related Art
A wide variety of machines have moveable members which are driven by a hydraulic actuator, such as a cylinder-piston arrangement or a hydraulic motor. For example, a backhoe has a tractor on which is mounted an assembly comprising a boom, an arm, an extender, and a bucket connected in series, with each of those components being driven by one or more cylinder-piston arrangement. That assembly swings left and right with respect to the tractor when driven by another cylinder-piston arrangement. The flow of fluid to and from each hydraulic actuator is controlled by a separate electrically operated valve assembly, which along with the actuator form a hydraulic function.
The hydraulic control is distributed throughout the machine by locating the valve assembly for a given hydraulic function in close proximity to the associated hydraulic actuator. Such distributed control reduces the amount of plumbing on the machine. A single hydraulic fluid supply conduit and a single fluid return conduit connect all the valve assemblies to the pump and tank on the tractor 18.
Today the hydraulic valve assemblies comprise one or more electrohydraulic valves which often are operated by a computer system. The operator in the cab of the backhoe manipulates joysticks or other input devices to generate electrical control signals for operating the valve assemblies located adjacent each hydraulic actuator. U.S. Pat. No. 6,718,759 describes a velocity based system for controlling a hydraulic system with multiple functions in which a velocity command is produced from a joystick signal. The velocity command and other signals for a given machine function are transmitted over a shared communication network to a separate function controller that is associated with the valve assembly which controls the hydraulic actuator for that machine function. Each function controller is located in close proximity to the associated valve assembly. The function controllers also send data and other messages over the communication network to the system controller.
Each hydraulic machine function in an electrically controlled hydraulic system has a parameter known as bandwidth. The bandwidth characterizes how well the hydraulic machine function responds to electrically commanded operations. As the frequency of the commands increase, a point occurs at which the hydraulics can no longer supply enough force to control the velocity to command signal frequency and thus the velocity begins to lag the command. That frequency point defines the bandwidth of the hydraulic machine function. The bandwidth of a given hydraulic machine function is dynamic being affected by different operating conditions, a significant one of which is the inertia of the load acting on the actuator. The greater the inertia, the higher the hydraulic force required to accelerate the function to a command velocity and thus the maximum acceleration that can be controlled is limited. The inertia of the boom assembly on a backhoe varies as the positions of the various components of that assembly change thereby altering the instantaneous bandwidth of the associated actuators. In addition the ability of the hydraulics to apply torque to this inertia changes with the position of the structures and also affects the instantaneous bandwidth of the associated actuators.
If the instantaneous bandwidth could be determined, the controller would be able adjust the commanded operation of the hydraulic actuators to achieve increased controllability. For example, attempting to operate the hydraulic actuator faster than it is able to respond, produces jerky motion of the associated machine component, however commanding the hydraulic actuator to operate slower than it is able, unnecessarily diminishes machine performance. Therefore, it is desirable to know the instantaneous bandwidth of each hydraulic machine function.
Controllability of the machine is paramount for achieving productive and efficient operation. Modern hydraulic systems have a variety of measured and sensed pressures that are used to enable or disable different orifices, pump modes, load sense pressures, and the like to optimize the machine performance in real time. This ensures that the hydraulic system controls acceleration and deceleration or machine components at rates that are acceptable to the operator over a wide range of component positions, loads and parameters induced from the operating environment. Inevitably, the variable component positions, loads, and environmental constraints require tradeoffs to be made and such tradeoffs often involve adding extra losses (damping) to the hydraulic system to ensure that the machine operates in a stable and predictable manner.
The transition from purely mechanical controls to electrohydraulic controls on heavy equipment, such as a backhoe, simply emulated the previous hydromechanical systems. Little work has been done to date to migrate some of the previous tradeoffs that were necessary. A missing part of the transition to electrohydraulic systems is not recognizing the capability of electronics and software to integrate velocity commands to create position indications for use in machine control. Although some machines today use position feedback from sensors for position or angle control, such as bucket leveling, these systems only modulate high level commands to the hydraulic functions based on a combination of such feedback signals and operator commands. Most hydraulic systems only modulate the valves or the pump given the same velocity or joystick command very similarly to hydraulic pilot pressure or spool position has been used.
It is desired to provide a method for estimating the position of different actuators and components on the machine and then use that information on a hydraulic function by function basis to remove some of the barriers that have existed in providing truly optimal control of the machine.
In the case of a backhoe, for example, the control of the swing function, which rotates the boom assembly, is affected by how far the boom, arm and extender project from the tractor. The closer that those components are to the tractor, the lower the swing inertia for the swing function. Likewise, the farther that the components of the boom assembly extend from the backhoe tractor, the greater the load that is placed on each hydraulic actuator associated with those components. Each of the boom, arm, extender and bucket of the boom assembly has a given mass that for each combination of stationary positions of those components produces a different static structure load on each hydraulic actuator. Thus, it is useful to know the position of the boom and arm with respect to the backhoe tractor to determine how to accelerate and decelerate the swing motion.